The present invention relates to resin-coated carriers used as a developer in combination with toner.
As electrostatic latent image development methods for electrophotography, two component development methods have been conventionally known in which development is carried out by transferring the developer to bring it into contact with an electrostatic latent image while the toner is frictionally charged by mixing an insulating non-magnetic toner with carrier particles.
One of the main function of the carrier is to frictionally charge the toner by making contact therewith.
To enhance the frictional charging ability of the carrier, it is an effective means to enlarge the surface area of the carrier, for example, by making the shape of the carrier irregular, forming fine concavities and convexities on the surface of the carrier or making its particle size small.
In magnetic materials such as ferrite used as the carrier, however, the so-called spent-toner phenomenon occurs when it is brought into contact with the toner to frictionally charge it for a prolonged period of time, because toner materials fuse on the surface of the carrier to reduce its effective surface area and therefore reduce its frictional charging ability. For this reason, the surface of the carrier is coated with a resin, which causes the degree of concavities and convexities on the carrier surface to be lessened, contrary to the purpose of enlarging the surface area. Thus, there remain the problems of durability, stability of charge for a prolonged period of time, and fogging of the toner unsettled. Use of a carrier with too small particle size causes some undesirable carrier phenomenon and adhesion of carrier, disadvantageously.
Recently color copying machines have been developed and carriers are used for the machines together with a light-transmittable color toner. Conventional charge controlling agents cannot be used in the light-transmittable color toner in such an amount as they have been used in a black toner because of the need of ensuring the light-transmittable property. This is due to the fact that the charge controlling agents are in most cases colored, which can impair the light-transmittable property of the toner. Accordingly, the carrier is required to have more advanced frictional charging characteristics in order to keep a certain level of charge amount with a less quantity of charge controlling agents.